Motion adaptive noise reduction is an effective video noise reduction method that is widely used. In such a conventional method, a motion detector is used to detect motion among the current video frame and its neighboring frames for every pixel in the video frame to generate a motion signal. The motion signal indicates the motion area and non-motion area within the current video frame, and is used to control temporal filtering for noise reduction. For the non-motion area, a temporal filter which takes the average of the corresponding pixels of the current frame and its neighboring frames, is utilized to reduce the video noise. For the motion area the temporal filter is switched off to avoid motion blurring.
A drawback of the conventional motion adaptive noise reduction method is what is know as a tailing effect, which appears as a noise tail following a moving object. The tailing is caused by switching off the temporal filter when the motion signal indicates motion. An existing method used in noise reduction system to handle the tailing effect is to apply a spatial filter to filter the motion area. When the motion signal indicates motion, instead of just switching off the temporal filter, the noise reduction system switches to a spatial filter.
However, due to different characteristics of the temporal filter and the spatial filter, when the filtered video sequence is viewed frame by frame, the temporal filtered area and the spatial filtered area look different. Further, when the filtered video sequence is played, flicker appears in the spatial filtered area because the residual noise of spatial filter at a given pixel changes from frame to frame. As such, simply switching from a temporal filter to a spatial filter can not solve the tailing problem, rather the tailing problem looks different and less obvious.